1. Field of the Invention
The present invention relates to an induction heating apparatus for a metal plate such as a steel plate or an aluminum plate. The present invention particularly relates to an induction heating apparatus that heats a metal plate by generating an induced current therein using an induction coil surrounding the metal plate. The present invention also relates to an induction heating apparatus, which is capable of heating a metal plate with high efficiency irrespective of the thickness of the metal plate and irrespective of whether the metal plate is magnetic or non-magnetic. The present invention is further capable of restraining overheating at an edge area of the metal plate.
2. Description of the Related Art
An indirect heating apparatus using a gas or electricity, or a direct heating apparatus using induction heating has been used for heating a metal plate to control the quality of the metal material in the heat-treatment process. Since a direct heating apparatus has no thermal inertia, unlike an indirect heating apparatus, a direct heating apparatus can save the time which is required by an indirect heating apparatus to reach a stable furnace temperature, and can easily control the heating rate, for example, when a thickness of plate is changed. Therefore, a direct heating apparatus does not require changing of the metal plate transportation speed, which prevents productivity from being lowered.
There are two types of induction heating apparatus for a metal plate. One type is an LF type (Longitudinal Flux type), in which a metal plate is heated by generating a circular induced current therein in the cross-section using an induction coil, where an alternate current with a frequency ranging normally from 1 KHz to 500 KHz is applied, surrounding the metal plate. FIG. 1 shows a schematic diagram of an LF type induction heating apparatus. FIG. 2 illustrates a circular induced current generated in the cross-section using an LF type induction heating apparatus. In FIG. 1, an induction coil 2 connected to an AC power supply 3 surrounds a metal plate 1. When a primary current 5 is passed through the induction coil 2, a flux 4 penetrates the metal plate 1 to generate an induced current around the flux 4. In FIG. 2, an induced current 6 generated in the cross-section of the metal plate 1 flows in an opposite direction to the primary current 5 running through the induction coils 2 which are located above and under the metal plate 1, respectively. The other type is a TF type (Transverse Flux type), in which induction coils with a core are located above and under the metal plate respectively. When an AC power supply to the coils is turned on, a flux penetrates the metal plate between the cores in the plate thickness direction to generate an induced current, which leads to beating of the metal plate.
In TF type heating, the induced current concentrates on a lateral end area of the metal plate and at the same time the current density in the vicinity of the end area is lowered, which easily causes a non-uniform temperature distribution in a lateral direction after heating. In particular, it becomes more difficult to provide a uniform heating when the positional relationship between the core of the induction coil and the metal plate is changed by shifting a width of the metal plate or by a snaking of the metal plate. In the background art, a technology that uses a rhombus-shaped coil was proposed so that the flux can always penetrate over an entire width of the plate by tilting the rhombus-shaped coil when the width of the metal plate is changed. However, this technology uses by leakage flux from the induction coil, which requires the metal plate and the induction coil to be close to each other. In addition, installation of a rotation mechanism on the induction heating apparatus where a large amount of current is supplied increases the difficulty in carrying out the technology on industrial scale.
The LF type heating is a method for heating a metal plate surrounded by an induction coil, which can make sure that a circular induced current is generated in the metal plate so as to heat the plate. An induced current that is generated in the cross-section of the metal plate in an LF type is concentrated at the depth “d” expressed in the following expression:d[mm]=5.03×10+5×(ρ/μrf)0.5  (1)
where d is the induced current penetration depth [mm], ρ is the specific resistance [Ωm], μr is the relative magnetic permeability, and f is the frequency [Hz] for heating.
An induced current penetration depth increases as a temperature of the metal increases because the specific resistance increases when the temperature of the metal increases. The relative magnetic permeability of ferromagnetic material or paramagnetic material decreases as the temperature becomes closer to the Curie point, and finally becomes 1 at a temperature above the Curie point. This means that the induced current penetration depth increases as the temperature increases. Since the relative magnetic permeability of a non-magnetic material is 1, its induced current penetration depth is larger compared to that of a magnetic material.
In LF type induction heating, if the induced current penetration depth is large and yet a thickness of the metal plate is thin, the induced current generated in an upper portion of the metal and the induced current generated in a lower portion of the metal cancel each other. This leads to heating that has a low efficiency.
For example, if a heating frequency of 10 KHz is used, the induced current penetration depth at room temperature is about 1 mm with aluminum of non-magnetic material, about 4.4 mm with stainless steel 304 (SUS304) and about 0.2 mm with steel of magnetic material. The current penetration depth of steel at temperature above the Curie point (at about 750° C.) is about 5 mm. Most steel plates for automobiles and home electric appliances, which are major commercial products that use metal plates, have a thickness of not more than 2 mm. Therefore, it is usually difficult to heat such metal plate with high efficiency without the induced currents in the upper and lower portions of the metal plate being canceled as mentioned above. It could be thought to increase the frequency of the AC current supplied to the LF type induction heating apparatus to several hundred KHz in order to make the depth of the induced current penetration shallower, so that canceling the induced currents can be avoided; however, it is not very practical to use a large current power source with such a high frequency on an industrial scale.
It has been proposed to use an induction heating apparatus that uses an induction coil surrounding a metal plate, which is capable of heating a metal plate with high efficiency even if the metal plate is at a high temperature and/or is a thin metal plate. In such induction heating apparatus, an induction coil located above the metal plate (upper induction coil) and another induction coil located below the metal plate (lower induction coil) are arranged parallel to each other, so as to be set respectively in different positions in a longitudinal direction of the metal plate. In other words, two projected images of the upper induction coil and the lower induction coil, which are respectively formed by vertically projecting the two induction coils onto the metal plate, are parallel to each other and in a different position in the longitudinal direction of the metal plate.
FIG. 3 is a schematic diagram of the above-mentioned induction heating apparatus where an induction coil 2a located above the metal plate 1 (upper induction coil) and another induction coil 2b located below the metal plate 1 (lower induction coil) are arranged parallel to each other and in a different position in the longitudinal direction of the metal plate. Reference numerals 7 and 8 represent a conductive member and an AC power supply 8, respectively. FIGS. 4A and 4B show the flow of the induced current in the metal plate 1 when the upper induction coil and the lower induction coil are arranged in a different position in the longitudinal direction of the metal plate. FIG. 4A is a schematic diagram illustrating the state of the induced current viewed from above the metal plate. FIG. 4B is a cross-sectional view taken on the line 4B-4B of FIG. 4A. Reference numeral 10 in FIG. 4A represents the flow of the induced current. When the upper induction coil and the lower induction coil are arranged so as to be set in a different position in the longitudinal direction of the metal plate, the upper path and the lower path of the circular induced current generated in the metal plate are also arranged to be set respectively in different positions in the longitudinal direction of the metal plate. Therefore, it makes it possible to heat the metal plate with high efficiency without cancellation of the induced currents in the upper and lower portions in the metal plate while the induced current penetration depth is large, even where the temperature of the metal plate is high and/or the metal plate is thin.
However, in the use of such an induction heating apparatus where the upper and lower induction coils are set in different positions in the longitudinal direction of the metal plate, an edge area of the metal plate in the width direction can become overheated compared to a central area of the metal plate in the width direction. This can result in a non-uniform temperature distribution as a finishing temperature in the transverse direction of the metal plate.
This phenomenon is experienced because a width of the induced current path in the edge area of the metal plate (corresponding to “d2” in FIG. 4a), where the current flows from an upper portion to a lower portion in the metal plate, is narrower than the induced current path in the upper and lower portions of the metal plate (corresponding to “d1” in FIG. 4A). Therefore, a current density in the edge area of the metal plate is higher than a current density in the central area. One reason for narrowing the current path in the edge area is that the current flowing in the edge area is to be shifted toward edge, so that the inductance between the induced current flowing in the edge area in the metal plate thickness direction and the primary current flowing through the induction coil arranged near the edge of the metal plate in the metal plate thickness direction can be lowered. Another reason for the overheating at the edge area is that the heating time at the edge area of the metal plate (defined as d3/(the traveling speed of the metal plate), where d3 is defined as in FIG. 4A) is longer than the heating time at the central area (defined as d1/(the traveling speed of the metal plate), where d1 is defined as in FIG. 4a).
Since a heat divergence by an induction heating apparatus is proportional to a square of the current density and the heating time, an edge area of the metal plate in the transverse direction is overheated compared to a central area of the metal plate in the use of such an induction heating apparatus where an upper induction coil and lower induction coil are respectively set in different positions so as to be away from each other in longitudinal direction of the metal plate.